I have been having low RPM performance issues with my 2.5V6 A6 Allroad. (See my other posts).
In desperation I went to the local stealership. Very seemingly knowledgeable chap took the car out. His diagnosis is the variable geometry turbo is not working a low engine speeds. He tested the actuator and confirmed it was working OK, but said that the fins inside the turbo could be faulty even thought the actuator works. Something about the variable pitch getting stuck?
When the car is ticking over the the actuator is in the lifted position, and it is revved up it drops, so looks like something is happening.
The problem is much worse when the car is cold.